The present invention is directed to bicycles and, more particularly, to an electronic shift control device for a bicycle transmission.
Bicycle transmissions include internal transmissions and external transmissions. Internal transmissions ordinarily include a planetary gear mechanism fitted in the rear wheel hub. External transmissions ordinarily have a multiple sprocket cassette mounted on the rear wheel hub or crank, and a derailleur guides a chain among the multiple sprockets. In either case, a shift control cable is connected to the transmission and to a manually operated shift controller mounted to the bicycle frame, and the rider operates the shift controller to select a desired transmission gear.
Recently, automatic bicycle transmissions have been developed which automatically shift the transmission to higher gear ratios when the bicycle is traveling at high speeds and which automatically shift the transmission to lower gear ratios when the bicycle is traveling at low speeds. Such automatic transmissions usually are equipped with an electric motor for operating the shift control cable, a speed sensor for sensing bicycle speed, a control element containing a microcomputer for controlling the electric motor in response to bicycle speed, and a case mounted to the bicycle frame for containing these components.
Dry cells and other primary cells, or nickel-cadmium and other secondary batteries, are used to supply power to the electric motor, control element, etc. Depending on the frequency of shifting, battery consumption may be heavy due to the relatively high power consumption of the electric motor. Batteries must be replaced or recharged when consumed, thus requiring time-consuming operations for the replacement or recharging. Another drawback is the need to constantly check remaining battery power and prepare reserve batteries, since shifting cannot be performed once the batteries are consumed.
The speed sensor ordinarily is mounted for detecting the passage of a magnet attached to the bicycle wheel, and a speed signal pulse is communicated to the control element for each wheel rotation. The control element shifts the bicycle transmission in response to the speed signal pulses. Since the speed sensor outputs one pulse per wheel revolution, the calculated bicycle speed and the actual bicycle speed may differ by up to one wheel rotation. Since the control unit operates in response to these speed signals, there is a delayed response that can make high-precision shifting control impossible. Furthermore, if the speed sensor is mounted in the case together with the other components, then the case must be disposed near the bicycle wheel, thus limiting possible device mounting positions.